i965: perform 2 uploads with dual slot *64*PASSTHRU formats on gen<8

The emission of vertex attributes corresponding to dvec3 and dvec4
vertex shader input variables was not correct when the <size> passed
to the VertexAttribL* commands was <= 2.

In 61a8a55f ("i965/gen8: Fix vertex attrib upload for dvec3/4
shader inputs"), for gen8+ we needed to determine if the attrib was
dual slot to emit 128 or 256-bit, independently of the VAO size.

Similarly, for gen < 8 we also need to determine whether the attrib is
dual slot to force the emission of 256-bits through 2 uploads.

Additionally, we make use of the ISL_FORMAT_R32_FLOAT format in this
second upload to fill these unspecified components with zeros, as we
also do for gen8+.

Fixes the following test on Haswell:
KHR-GL46.vertex_attrib_binding.basic-inputL-case1

v2: Added more inline comments to explain why we are using
    ISL_FORMAT_R32_FLOAT and its consequences, as requested by
    Alejandro and Antía.

Fixes: 75968a66 ("i965/gen7: expose OpenGL 4.2 on Haswell when
supported")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103006


Cc: Alejandro Piñeiro <apinheiro@igalia.com>
Cc: Juan A. Suarez Romero <jasuarez@igalia.com>
Cc: Antia Puentes <apuentes@igalia.com>
Cc: Rafael Antognolli <rafael.antognolli@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Antia Puentes <apuentes@igalia.com>
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 5a7aba2e)